How to add item to my inventory when I pick them up?

I'm somewhat new to Unity and game development but I keep getting this error:

 NullReferenceException: Object reference not set to an instance of an object
 HUD.InventoryScript_ItemAdded (System.Object sender, .InventoryEventArgs e) (at Assets/HUD.cs:21)
 inventory.AddItem (IInventoryItem item) (at Assets/Scripts/inventory.cs:42)
 collider.OnCollisionEnter (UnityEngine.Collision col) (at Assets/Scripts/collider.cs:38)

Usually when I get a NullReferenceException, the fixes is dragging in the object that a script is referring to. From what I can see here, the collider (which is on the player) is detecting the collision between the player and the item. It sent the information of collision object to the inventory class which adds the object to the List of inventory items. From here things get a bit murky as it throws the error in the HUD.InventoryScript_ItemAdded method.

Here is the HUD.cs script

 using UnityEngine;
 using UnityEngine.UI;

 public class HUD : MonoBehaviour {

     public inventory Inventory;


     // Use this for initialization
     void Start () {

         Inventory.ItemAdded += InventoryScript_ItemAdded;
     
     }

     private void InventoryScript_ItemAdded(object sender, InventoryEventArgs e)
     {
         Transform inventoryPanel = transform.Find("InventoryPanel");
         foreach(Transform slot in inventoryPanel)
         {

             //Border to image
             Image image = slot.GetChild(0).GetChild(0).GetComponent<Image>();

             //We found the empty slot
             if (!image.enabled)
             {
                 image.enabled = true;
                 image.sprite = e.Item.Image;

                 //TODO: Store reference to the item

                 break; //what does this do?
         }
     }
  }
 

 

 // Update is called once per frame
 void Update () {
     
 }
 }  

This is the inventory script

 public class inventory : MonoBehaviour {

 //fixed number of slots
 private const int SLOTS = 10;

 //creates an arraylist of inventory items
 private List<IInventoryItem> mItems = new List<IInventoryItem>();

 //handles event?
 public event EventHandler<InventoryEventArgs> ItemAdded;

 //adding item method
 public void AddItem(IInventoryItem item)
 {

     //if there are free slots available
     if (mItems.Count < SLOTS)
     {

         //collide with item
         Collider collider = (item as MonoBehaviour).GetComponent<Collider>();

         
         if (collider.enabled)
         {
             //if the collider of the object is enabled turn it off
             collider.enabled = false;

             //add to array list
             mItems.Add(item);

             item.OnPickup();

             //all subscribers of event are notfified?
             if(ItemAdded != null)
             {
                 ItemAdded(this,new InventoryEventArgs(item));
             }
         }
     }
 }
 }

And finally this is the collider script I put on my player

 public class collider : MonoBehaviour
 {
     public inventory Inventory;
     //public GameObject pickupEffect;
     public gun g;

     void OnTriggerEnter(Collider other)
 {
     
     if(other.gameObject.tag == "item")
     {
         Debug.Log("You Hit an item");
         Destroy(other.gameObject);

     }

     if(other.gameObject.tag == "ammo")
     {
         Debug.Log("Picked up ammo");
         Destroy(other.gameObject);
         g.ammo += 30;
     }
 }

 void OnCollisionEnter(Collision col)
 {
     
     if(col.gameObject.tag == "collectable")
     {
         Debug.Log("This object is collectable");
         IInventoryItem item = col.gameObject.GetComponent<IInventoryItem>();
         if (item != null)
         {
             Inventory.AddItem(item);
             Debug.Log("Collided with an inventory item");
         }
     }

    
 }

 /*
  * 
 //check if item collided with has an inventory item interface
 private void OnControllerColliderHit(ControllerColliderHit hit)
 {
     Debug.Log("Collided with an inventory item");
     IInventoryItem item = hit.collider.GetComponent<IInventoryItem>();
     if(item != null)
     {
         Inventory.AddItem(item);

         Debug.Log("Collided with an inventory item");
     }
 }

 */

}

Sorry if the code is not formatted correctly but the system for creating code in this forum seems to be a bit tedious because you have to space 4 times for each line.

Anyway, why am I getting this error and how could I fix it while implementing this code?